Merge statement is designed to simplify/optimize final step of ETL processing for data warehousing, but when I tried to use it, I found that existing Firebird implementation is nearly impossible to use for this purpose.
In the situation when source data is mostly unchanged Firebird's MERGE logic triggers expensive row updates and recalculation of aggregates for unchanged rows.

The request is to implement SQL2008 MERGE syntax with DELETE extension:

<merge statement> ::=
MERGE INTO <target table> [ [ AS ] <merge correlation name> ]
USING <table reference>
ON <search condition> <merge operation specification>

<merge correlation name> ::=
<correlation name>

<merge operation specification> ::=
<merge when clause>...

<merge when clause> ::=
<merge when matched clause>
| <merge when not matched clause>

<merge when matched clause> ::=
WHEN MATCHED [ AND <search condition> ]
THEN <merge update specification>

<merge when not matched clause> ::=
WHEN NOT MATCHED [ AND <search condition> ]
THEN <merge insert specification>

<merge update specification> ::=
UPDATE ...
| DELETE

DELETE is the useful extension to standard MERGE syntax supported by DB2.